Our overall goal is to develop protocols by which oncolytic viruses can be systemically delivered in patients leading to treatment of metastatic tumors. Based upon our pre-clinical experiments showing that intravenous (i.v.) delivery of oncolytic reovirus can have significant anti tumor activity in animal models, we have completed Phase I/II clinical trials, confirming that i.v. reovirus is safe in humans. Using data from these trials, our additional pre-clinical experiments showed that tumor vasculature can be conditioned for increased reovirus replication following i.v. delivery by carefully timed combination of paclitaxel chemotherapy and reovirus virotherapy, leading to a Phase I trial of carboplatin/paclitaxel and reovirus for relapsed/metastatic cancers. During the course of this trial, we observed very encouraging suggestions of possible clinical effects, as well as the emergence of aggressive recurrences in some patients who initially responded very well following the therapy. In the current proposal, we will build further on these pre-clinical and clinical data, which have, cumulatively, shown that PAC/Reo is a well tolerated, and potentially efficacious, method of delivering oncolytic virotherapy to tumor bearing patients. Therefore, the overall hypothesis of the current proposal is that it will be possible to develop novel treatments which both improve initial tumor responses to i.v. reovirus and either prevent, or treat, aggressive tumor recurrences. To test this hypothesis, we will improve initial anti tumor responses following i.v. reovirus, in virus immune/ non-immune mice, by conditioning the host with cytokines, chemo- or radio-therapy (Aim 1) and/or chaperoning, and protecting, i.v. administered reovirus using immune cell carriers (Aim 2), in order to enhance levels, and immune consequences, of virus delivered to the tumor and/or its vasculature. In addition, we will treat recurrent tumors, which fail initial systemic virotherapy, with rational, mechanism-based 2nd line therapies by targeting the predictable recurrent tumor phenotype which emerges upon treatment failure with front line chemo-/virotherapy (Aim 3). Overall these experiments will lead to new clinical trials for i.v. delivery of reovirus and should also be directly applicable to a range of both different tumor types and oncolytic viruses.